It is often desirable to understand accurate size distributions, spatial distributions, and number densities of particles within a flow, and more specifically, of cloud particles. For example, scientists continually seek a better understanding of how local cloud particle size distributions vary inside cloud regions by cloud age and type, how cloud particles are spatially distributed on sub-centimeter scales due to mixing, entrainment, and turbulent processes, and how ice and liquid water particles are spatially distributed or partitioned within mixed-phase regions of a cloud. This type of information is useful in understanding and modeling cloud processes such as precipitation formation and radiative transfer and for validation of remote sensing and satellite measurements.
Many methods and instruments have been devised to measure cloud particles, yet there remains considerable uncertainty in measuring small particles between approximately 10 μm and 100-200 μm using standard in-situ imaging instrumentation (e.g., aircraft imaging probes). Imaging instrumentation images particles appearing at the focus of a laser sheet by using a linear photodiode array to detect intensity levels of light impinging on the linear array (e.g. light that has been scattered by the particles or light that has not been absorbed by the particles). Because small particles have a small depth of focus, conventional imaging probes have correspondingly low sample volumes and volume sample rates. That is, there is only a small area of focused light through which particles to be imaged may appear.
Beyond a small depth of focus, there is significant uncertainty in the depth of focus for small particles due to variances in optical alignment and the relative velocity between the instrumentation and the particles to be imaged. This depth of focus uncertainty causes uncertainty in sample volume size, which translates to uncertainty in particle concentrations. This occurs because particles outside the depth of focus of traditional imaging probes appear blurry or fainter and larger in size than the focused imaged particle, resulting in over-estimation of particle sizes. Further, out-of-focus particles can appear fragmented which leads to miscounting and under-sizing of the real particle population. See Korolev, A.: Reconstruction of the Sizes of Spherical Particles from their Shadow Images, Part I: Theoretical Considerations, J. Atmos. Ocean. Tech., 24, 376-389, 2007.790. The depth of focus issues associated with imaging small particles also lead to particle size uncertainty. Namely, while traditional imaging probes only image particles appearing within the depth of focus of the laser sheet, the exact location of a particle within the depth of focus is unknown. Because any distance between the particle to be imaged and the exact focal point of the laser sheet introduces error into the particle image, there is uncertainty regarding the actual size of imaged particles.